1. Field of the Invention
The present invention relates to a vibration isolating apparatus which is used in a vehicle, an ordinary industrial machine or the like, and which absorbs vibration from a vibration generating portion.
2. Description of the Related Art
A vibration isolating apparatus serving as an engine mount is disposed between the engine and the vehicle body of a vehicle such as an automobile, and prevents the transmission of the engine vibration to the vehicle body.
Types of vibrations generated at the engine include so-called shake vibration which is generated when the vehicle is traveling at high speeds or the like, and so-called idle vibration which is generated when the vehicle is idling or is traveling at speeds of around 5 km/h.
The respective frequencies of shake vibration and idle vibration are different; generally, the frequency of shake vibration is less than 15 Hz, whereas the frequency of idle vibration is 20 to 50 Hz.
Fluid-filled vibration isolating apparatuses have been proposed as vibration isolating apparatuses which absorb and reduce shake vibration and idle vibration.
Such a fluid-filled vibration isolating apparatus is equipped with a main fluid chamber and a plurality of auxiliary fluid chambers. Portions of the wall surfaces of the main fluid chamber and the auxiliary fluid chambers are formed by an elastic body. The main fluid chamber and the auxiliary fluid chambers are connected by a plurality of restricting passages having respectively different sizes. Further, as is well-known, this type of vibration isolating apparatus reduces vibration by the fluid moving reciprocally within the restricting passages or resonating within the restricting passages when vibration is input.
The auxiliary fluid chamber of the vibration isolating apparatus is structured such that a diaphragm, whose opposite side contacts air, is a portion of a partitioning wall. Specifically, due to the diaphragm deforming, the volume of the auxiliary fluid chamber can be varied, and the fluid within the restricting passage communicating the main fluid chamber and the auxiliary fluid chamber can be made to move reciprocally. As a result, if the motion of the diaphragm is restricted, the fluid cannot flow within the restricting passage.
On the basis of this principle, fluid-filled vibration isolating apparatuses have been proposed in which the suction vacuum generated by the intake system of the engine is used to vary the isolating characteristic. In such a vibration isolating apparatus, an air chamber is provided at one surface side of the diaphragm, and the air chamber is connected to the intake system of the engine (e.g., an intake manifold) via a two-port/three-position switching valve (e.g., an electromagnetic valve). Accordingly, by communicating the intake system of the engine and the air chamber by the two-port/three-position switching valve, the internal pressure of the air chamber is lowered by the vacuum of the intake system of the engine, and the diaphragm is made to fit tightly to the inner wall of the air chamber.
In this way, fluid moves reciprocally in the restricting passages other than the restricting passage which is connected to the auxiliary fluid chamber facing the diaphragm which fits tightly to the inner wall, and an isolating effect is obtained.
However, the above-described vibration isolating apparatus has the following drawbacks.
First, a piping hose for supplying the vacuum generated by the engine intake system to the vibration isolating apparatus is needed. The piping hose results in added costs. Further, an assembly process for guiding and fixing the piping hose within the engine room is necessary, which results in an increase in manufacturing costs.
Second, in the event that the diaphragm breaks, there is the concern that the fluid within the vibration isolating apparatus will penetrate into the engine through the piping hose for the vacuum.
Third, when the diaphragm is sucked and is fit tightly to the inner wall of the air chamber due to the vacuum of the engine intake system, the fluid within the main fluid chamber moves to the auxiliary fluid chamber as the diaphragm moves. As a result, the volume of the main fluid chamber decreases, the elastic body caves in, and the position of a mounting member which is mounted to the elastic body changes. Specifically, there is a drawback in that the dimension between the vibration receiving portion and the vibration generating portion changes. For example, if the engine is supported at the vehicle body via the vibration isolating apparatus, the height of the engine varies.
Even in cases in which such a vibration isolating apparatus is used, when the engine starts, the idle rotational frequency of the engine increases and becomes a rotational frequency (for example, 900 to 1200 rpm) which is greater than or equal to the rotational frequency (around 700 rpm as illustrated in FIG. 13) which is set as the idle rotational frequency. Therefore, the vibration frequency becomes higher than ordinary idle frequency.
As a result, even if the vibration isolating apparatus is tuned in advance so as to match idle vibration, when the engine starts, the dynamic spring constant K increases, and vibration in the antiresonant region of the fluid is generated. A drawback arises in that the vibration cannot be reduced until the rotational frequency falls to less than or equal to the idle rotational frequency.